1. Field of the Invention
The present invention relates to a method of measuring the thickness of an epitaxial layer, and more particularly to an in-line method of measuring the thickness of an epitaxial layer.
2. Description of the Related Art
As is well known, it is important in the fabrication of semiconductor devices to know the thickness of an epitaxial layer on a semiconductor wafer. Different methods are known within the prior art for measuring or determining the thickness of an epitaxial layer including methods based upon IR interference physical optic theory. In accordance with such theory, IR energy is directed onto a wafer and is reflected from the surface of the epitaxial layer and from the interface between the epitaxial layer and the underlying substrate. The IR energy is directed as an incident beam onto a small area of the wafer at a position where the epitaxial thickness is to be measured. Such incident beam is divided to form two reflected beams. One beam is reflected from the surface of the epitaxial layer and the other beam is reflected from the epitaxial layer/substrate interface. The two reflected beams interface with each other in such a manner that the epitaxial thickness can be determined by spectral reflectance and interferometric methods.
The spectral reflectance method is based on the phenomena that the degree of optical interference between the two reflected beams cyclically varies at each wavelength across a spectrum. The variation produces a series of maxima and minima reflectance values in accordance with the degree of constructive and destructive interference at the different wavelengths. Such method generally involves measuring the spectral reflectance and then calculating the thickness using the reflectance at two different maxima or minima.
In the interferometric method, an interferometer is used to generate an interferogram from the two reflected beams. The interferogram includes a center burst or peak and two side bursts or peaks created as a result of displacement of the interferometer mirror. In a perfect system, the interferometer would be symmetrical and the degree of mirror displacement between two positions corresponding to two of the bursts or peaks, is proportional to the epitaxial thickness. In actual practice however, the interferogram is asymmetrical and a double Fourier Transform and other mathematical manipulations are performed to create an idealized interferogram from which the thickness is calculated as a function of mirror displacement between the side peaks.
However, when the difference of doping concentrations between the epitaxial layer and the substrate which are the same material such as silicon is negligible or the doping concentration of the epitaxial layer is lower than the doping concentration of the substrate, the optical methods such as the Fourier-Transform Infrared Spectroscopy (FTIR) method set forth and other conventional electrical methods such as the four point probe method will not be suitable anymore.
Therefore, it is necessary to provide a new method to solve the problems mentioned above, and the method of the present invention is just the one.
It is therefore an object of the invention to provide a new method of measuring the thickness of an epitaxial layer that are always available particularly when the doping concentration of the epitaxial layer is lower than or the same as the doping concentration of the substrate.
It is another object of this invention to provide a new in-line method of measuring the thickness of an epitaxial layer when the materials of the epitaxial layer and the substrate are the same.
It is a further object of this invention to provide a new method of measuring the thickness of an epitaxial layer and the growth rate ratio of a polycrystal layer to a single crystal layer.
To achieve these objects, and in accordance with the purpose of the invention, the invention uses a method of measuring the thickness of an epitaxial layer. The method comprises the steps: an ousubstrate having a non-single crystal layer thereon, wherein the non-single crystal layer only covers a portion of the substrate; forming an epitaxial layer over the substrate and the non-single crystal layer, wherein the portion of the epitaxial layer on the non-single crystal layer grows into a polycrystal layer; measuring the thickness of the polycrystal layer and the thickness of the non-single crystal layer; and measuring the thickness difference between the polycrystal layer plus the non-single crystal layer and the epitaxial layer, thereby the thickness of said epitaxial layer equals to the result of the total thickness of the polycrystal layer plus the non-single crystal layer minus the thickness difference between the polycrystal layer plus the non-single crystal layer and the epitaxial layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.